1. Field of the Invention
This invention relates to magnetic media for recording and, in particular, to a flexible tape with dual magnetic surfaces.
2. Description of the Related Art
With the ever increasing demand for miniaturization, greater recording speed, and larger storage capacity, it has been a long-term objective of the magnetic recording industry to be able to record magnetic information on both surfaces of the magnetic medium of interest. That goal has been accomplished satisfactorily with disks, but is has been difficult to implement it with flexible-tape magnetic media. A problem for wound spools of tape, as compared to disks, arises from the very thin substrate used to separate the magnetic layers, which allows what in the art is referred to as a “print-through” or “ghost” imaging of the data pattern from one surface onto the opposite surface of the tape. This condition results from magnetic flux lines passing through the recording layer and the substrate during the recording process with sufficient strength to also record a poorly resolved signal on the magnetic coating on the opposite side of the tape. The use of a thicker tape substrate reduces print-through, but it also reduces the volumetric storage capacity of the tape. Thus, the art is evolving in the direction of ever thinner tapes and thinner recording layers (typically less than 0.4 microns).
In the recording process, a strong magnetic field is preferably applied to a very shallow depth in order to minimize penetration of the field flux through the substrate and, if present, through the coating on the opposite side of the tape. A useful tool for improving the effects of a strong magnetic field past the recording layer has been the use of a weak magnetic underlayer as a “keeper” or shunt for the field, thereby preventing signal penetration. Several patents have disclosed this approach to improve signal performance in the magnetic layer of thin-substrate tapes. See, for example, U.S. Pat. Nos. 4,109,046, No. 4,423,454, No. 4,200,678, and No. 4,075,384. None of these patents, though, addressed the use of weak magnetic underlayers to enable dual-surface recording.
Another problem for spools of dual-surface tape lies in the fact that the two magnetized layers are placed in close proximity to one another when the two surfaces of the tape are spooled into a wound pack of tape. As a result of the winding process, the magnetic medium on one side of the tape is necessarily placed physically against the magnetic medium on the other side found from the previous wrap of the tape on the reel. This condition produces what in the art is referred to as “contact recording”; that is, imaging of the data pattern on one surface onto the opposite surface of the tape. Moreover, when two recording surfaces are placed in direct contact with one another, errant magnetization on either side of the tape may also result.
Various implementations of dual-surface magnetic tapes are described in the prior art, such as in Japanese Patents No. JP54050304, No. JP4163715, and No. JP5041844, but none of these inventions addressed the print-through and contact-recording problems described above. Other prior-art disclosures described inventions directed at improving the performance of double-sided magnetic tapes. For example, U.S. Pat. No. 3,725,611 describes a double-coated magnetic tape where the two ferromagnetic layers on opposite sides of the tape have different thicknesses designed to improve the recording efficiency of short and long wavelength signals from a master tape. Since both sides of the tape are utilized for a single recording operation, though, this invention does not improve the storage capacity of the medium.
Similarly, U.S. Pat. No. 5,850,328 describes a two-sided magnetic recording tape where the magnetic particles are aligned in differing magnetic orientation during the manufacturing process. The magnetic orientation of each side is of sufficient angular displacement with respect to the other side to minimize the effects of contact recording and print-through. The invention requires the use of special read/write heads in azimuth alignment with the tape.
Because of the mentioned special requirements and shortcomings, none of the solutions heretofore described in the prior art has gained commercial acceptance. Nonetheless, as tape thickness is progressively reduced to afford more storage capacity, the use of double-sided tape has become an ever attractive objective and there is still a need for a practical implementation of the dual-surface magnetic-tape concept. This invention provides a simple solution based on the discovery of new combinations of properties in the magnetic and substrate layers that are particularly suitable for dual application with thin-film tape substrates.